This invention relates to methionine.sup.5 -enkephalin sulfoxides and sulfones which display agonist activity at opiate receptors. These sulfoxides and sulfones are derivatives of methionine.sup.5 -enkephalin, one of the two components of naturally occurring enkephalin. Methionine.sup.5 -enkephalin is represented by the following formula ##STR1## wherein the Tyr, Phe and Met amino acid residues are all of the L-stereochemical configuration.